This disclosure relates generally to methods and apparatus for actively cooling downhole electronics or other component contained within a downhole tool.
Increasingly hotter bore holes (wells) are being encountered in the oil and gas and geothermal industries. Oil and gas wells of 400 F have been encountered in Texas, North Sea, Thailand, and other parts of the world. Geothermal holes are 500 to 600 F. Most commercial available electronics are typically limited to ˜250 F maximum. A few electronics have been pushed to high temperatures but the majorities are low temperature. All it takes is one component to be rated at 250 F out of the many other components to have the whole electronics package rated to 250 F. Many electronics suffer drift at elevated temperatures and lose accuracy. Electronic components rated to 400 F will experience shortened life due to the degrading effects of high temperatures. One way to get around these temperature dilemmas is to cool the tool that houses the electronics thus cooling the electronics. The electronics (often referred to as the payload) is often an assembly of many electrical components typically mounted on a printed circuit which is typically mounted on a chassis. Sometimes the electronics consist of an electrical sensor or sensors mounted directly to the chassis and/or housing.
Methods used to cool downhole tools in a high temperature environment can be broadly classified as either passive or active systems. Passive systems have a finite operating time. Passive systems typically start with a cooled tool and provide ways and means to retard (slow down) the heating up of the tool to allow enough time for the tool to complete its job before the tool exceeds its temperature limit. Thermal insulation and devices such as Dewar flasks are a common way to achieve this. Eutectic (phase change) materials and heat sinks are another. However, the time duration is usually only several hours. This is OK for some wireline tools which are tripped into and out the well in a matter of several hours, but this is not good for longer duration wireline tools or drilling tools that are required to stay in the well for several days at a time.
Some passive systems can extend this time by pre-cooling heat sinks (typically in liquid nitrogen) before tripping downhole. Another way is to transport coolants or chemicals downhole to cool the tool but without a way to rejuvenate these materials downhole the time is still limited. The time can be extended by transporting more materials downhole but the large volume requirements make this impractical.
An active system uses work to pump heat out of the tool and into the surrounding environment. This requires power downhole and as long as there is power this cycle go on forever (assuming parts did not wear out). This power is typically derived from the drilling fluid (mud) being continuously circulated in and out of the well, electrical power conducted through a wireline, and/or stored power such as batteries.
Active systems are required for multiple days downhole (i.e. during the drilling process). There are many active systems such as vapor compression refrigeration, Brayton, absorption, Joule-Thompson, thermoacoustic, thermoelectric, magnetocaloric, electrocaloric, etc. Gloria Bennett (Los Alamos National Laboratory) published the pros and cons of these systems in 1988 in her paper Active Cooling for Downhole Instrumentation: Preliminary Analysis and System Selection. The vapor compression refrigeration cycle has many advantages. It is one of the more efficient systems. It has been in use since the early 1800's and is found in refrigerators, homes, buildings, industrial plants, cars, etc. It is a very well understood, simple, and durable system. Coolant can be selected to fit almost any range of temperatures.
Thus, there is a continuing need in the art for methods and apparatus for actively cooling downhole electronics or other component contained within a downhole tool.